This invention relates to the field of image projectors. More particularly, this invention relates to the field of angled illumination for a single order grating light valve based projection system.
In recent years, light modulators have been developed using MEMS (micro-electro-mechanical systems) technology in which moveable elements are configurable to direct light. An example of such light modulators is a grating light valve type device (GLV type device) taught in U.S. Pat. No. 5,311,360 to Bloom et al., in which the GLV type device is configurable in a reflecting mode and a diffracting mode. The GLV type device taught by Bloom et al. is isometrically illustrated in FIG. 1. The GLV type device 10 includes moveable elongated elements 12 suspended over a substrate 14.
A first side view of the GLV type device 10 of the prior art is illustrated in FIG. 2A, which shows the GLV type device 10 in the reflecting mode. The moveable elongated elements 12 each include a first reflective coating 16. Interspersed between the moveable elongated elements 12 are second reflective coatings 18. In the reflecting mode, upper surfaces of the first and second reflective coatings, 16 and 18, are separated by a height difference of a half wavelength xcex/2 of incident light I. The incident light I reflecting from the second reflecting coatings 18 travels a full wavelength further than the incident light I reflecting form the first reflecting coatings 16. So the incident light I, reflecting from the first and second reflecting coatings, 16 and 18, constructively combines to form reflected light R. Thus, in the reflecting mode, the GLV type device 10 produces the reflected light R.
A second side view of the GLV type device 10 of the prior art is illustrated in FIG. 2B, which shows the GLV type device in the diffracting mode. To transition from the reflecting mode to the diffracting mode, an electrostatic potential between the moveable elongated elements 12 and the substrate 14 moves the moveable elongated elements 12 to contact the substrate 14. To maintain the diffracting mode, the electrostatic potential holds the moveable elongated elements 12 against the substrate 14. In the diffracting mode, the upper surfaces of the first and second reflective coatings, 16 and 18, are separated by a quarter wavelength xcex/4 of the incident light I. The incident light I reflecting from the second reflecting surfaces 18 travels a half wavelength further than the incident light I reflecting from the first reflective coatings 16. So the incident light I, reflecting from the first and second reflecting coatings, 16 and 18, destructively interferes to produce diffraction. The diffraction includes a plus one diffraction order D+1 and a minus one diffraction order Dxe2x88x921. Thus, in the diffracting mode, the GLV type device 10 produces the plus one and minus one diffraction orders, D+1 and Dxe2x88x921.
A first alternative GLV type device of the prior art is illustrated in FIGS. 3A and 3B. The first alternative GLV type device 10A includes first elongated elements 22 interdigitated with second elongated elements 23. The first elongated elements 22 include third reflective coatings 26; the second elongated elements 23 include fourth reflective coating 28. In the reflecting mode, illustrated in FIG. 3A, the third and fourth reflective coatings, 26 and 28, are maintained at the same height to produce the reflected light R. In the diffracting mode, illustrated in FIG. 3B, the first and second reflected coatings, 26 and 28, are separated by the second height difference of the quarter wavelength xcex/4 of the incident light I to produce the diffraction including the plus one and minus one diffraction orders, D+1 and Dxe2x88x921.
A display system utilizing a GLV type device is taught in U.S. Pat. No. 5,982,553 to Bloom et al. The display system includes red, green, and blue lasers, a dichroic filter group, illumination optics, the GLV type device, Schlieren optics, projection optics, a scanning mirror, and display electronics, which project a color image onto a display screen. The red, green, and blue lasers, driven by the display electronics and coupled to the GLV type device (via the dichroic filter group and the illumination optics) sequentially illuminate the GLV type device with red, green, and blue illuminations. The GLV type device, driven by the display electronics, produces a linear array of pixels which changes with time in response to a signal from the display electronics, each pixel configured in the reflecting mode or the diffracting mode at a given instant in time. Thus, the GLV type device produces sequential linear arrays of red, green, and blue pixels with each of the red, green, and blue pixels in the reflecting mode or the diffracting mode.
The red, green, and blue pixels are then coupled to the Schlieren optics which blocks the reflecting mode and allows at least the plus one and minus one diffraction order, D+1 and Dxe2x88x921, to pass the Schlieren optics. Thus, after passing the Schlieren optics, the linear arrays of the red, green, and blue pixels have light pixels corresponding to the pixels at the GLV type device in the diffracting mode and dark pixels corresponding to pixels at the GLV type device in the reflecting mode. The projection optics (via the scanning mirror) project the linear arrays of the red, green, and blue pixels onto the display screen while the scanning mirror, driven by the display electronics, scans the linear arrays of the red, green, and blue pixels across the display screen. Thus, the display system produces a two dimensional color image on the display screen.
An alternative display system utilizing the GLV type device includes the red, green, and blue lasers; red, green, and blue illumination optics; first, second, and third GLV type devices; the dichroic filter group; the projection optics; the scanning mirror; and the display electronics. The red, green, and blue lasers, via the red, green, and blue illumination optics, illuminate the first, second, and third GLV type devices, respectively. The first, second, and third GLV type devices produce the linear arrays of the red, green, and blue pixels, respectively, in response to signals from the display electronics. The dichroic filter group directs the light from the linear arrays of the red, green, and blue pixels to the Schlieren optics, which allows at least the plus one and minus one diffraction order, D+1 and Dxe2x88x921, to pass the Schlieren optics. The projection optics, via the scanning mirror, project the linear arrays of the red, green, and blue pixels onto the display screen while the scanning mirror, driven by the display electronics, scans the linear arrays of the red, green, and blue pixels across the display screen. Thus, the alternative display system produces the two dimensional color image on the display screen.
Examples of applications for a GLV type device base display system include a home entertainment system, a boardroom application, and a cinema application among others. In the home entertainment system or the boardroom application, the GLV type device based display system projects the two dimensional color image onto the display screen located on a wall. In the cinema application, the GLV type device based display system projects the two dimensional color image from a display booth onto a cinema screen.
A GLV type device based display may also be utilized in printing applications. In such a case, the system would not include a scanning mirror, and the printing media, replacing a screen, would move to effectuate printing from a fixed line of light.
The aforementioned GLV type device based display systems put light in the xc2x11 diffraction orders. Theoretically, when light is filtered into two diffraction orders, the maximum amount of light that can be transmitted or reflected is equal to only 81% of the incident light beam. Another problem encountered in this type of system is the need for a more complex separating optics configuration or Schlieren optics. In such a system that filters light into two separate diffraction orders, a separating optical system must have two slits to receive the two orders. This configuration requires a complicated set of separating optics to properly separate the two orders.
Yet another disadvantage to implementing a GLV type device based system such as this is the requirement of the GLV type device producing a wide cone of light. In a system that produces light in the xc2x11 diffraction orders, all of the optics between the GLV type device and the projection screen must have a low F number in order to collect a large amount of light. This means that the optics must have a high optical throughput, thus requiring a larger lens. This larger lens captures more light, including additional background light, thus producing an image with a lower contrast, thus a less clear picture. Additionally, a larger lens means greater expense.
What is needed is a display system that implements a diffracted light modulator that puts light in a single diffraction order while providing a higher contrast. This system would allow a larger percentage of the incident light to be put in a diffraction order. A light modulator utilizing only one diffraction order would also allow for a less complex and expensive separating optics configuration. Additionally, utilizing such a light modulator would eliminate the need for all of the optics to have a low F number and high optical throughput, thereby reducing the cost of the entire system.
The present invention is a display apparatus and method for providing angled illumination for a single order grating light valve projection system. The display apparatus and method includes a light modulator being optically coupled to illumination optics such that in operation the illumination optics illuminate the light modulator with an off-axis illumination and further such that in operation the light modulator directs light onto an optic axis for a bright pixel, thereby forming on-axis light. Further, the light modulator directs the light away from the optic axis for a dark pixel, thereby forming off-axis light.
The display apparatus and method for providing angled illumination for a single order grating light valve projection system also includes separating optics that are optically coupled to the light modulator such that in operation they separate the off-axis light from the on-axis light, where the on-axis light produces a two dimensional image that is in the preferred embodiment a real image. Alternatively, the two dimensional image is a virtual image.
Lastly, the apparatus and method includes projection and scanning optics that are optically coupled to the separating optics.